1. Field of the Invention
The present invention relates to a process for producing a metal-coated, metallized component of aluminum nitride ceramic, a metallizing paste for carrying out the process and also the metal-coated component obtained thereby.
2. Description of Related Art
In electronics it is prior art to metallize components of ceramic materials containing mostly aluminum oxide or aluminum nitride with high-melting metals and then to further reinforce these metallized components by application of further metal layers. Further components such as diodes, resistors and the like can then be applied to these uppermost metal layers by means of soft soldering, for example in the production of electronic circuits. High-melting metals (refractory metals) used are preferably tungsten and molybdenum. The uppermost metal layers can then be deposited on the metallized components, preferably by means of electrolytic processes.
In microelectronics, aluminum nitride components, especially substrates, are preferably used because of their highly thermally conductive properties. For this purpose, no SiO.sub.2 -containing glass phase is added as sintering aid to the aluminum nitride powder prior to firing, as is known in the production of aluminum oxide components. On the other hand, it is known in the prior art that a firmly adhering metallization with high-melting metals such as tungsten or molybdenum is only possible if certain amounts of SiO.sub.2 -containing glass phase (at least 1.5% by weight) are present in the fired ceramic (Bronnes, R. L., Die Herstellung geloteter Keramik-Metallverbindungen mit Kathodenzerstaubung als Metallisierungstechnik, Philips Technische Rundschau 35, 227 (1975) and Cooper, D., The Development of High Quality Alumina Substrates, Industrial Ceramics 9, 115 (1989)). If the proportion of the glass phase is too low, it is also possible, as an alternative, to mix an addition of glass into the metallizing paste (Fulrath, R. M., Ceramic Bulletin 47, 493 (1968)). However, when using components of aluminum nitride ceramic it has to be noted that aluminum nitride is thermodynamically unstable toward many oxidic compounds, for example ZnO, PbO, Bi.sub.2 O.sub.3, B.sub.2 O.sub.3. Lowering of the free enthalpy is achieved by the formation of aluminum oxide, metal and nitrogen in a solid-state reaction. This reaction results in the destruction of the particle bonding in the ceramic, which lowers the strength (Kageyama, M., IMC Proceedings 1988, 161 (1988) and Kubota, T., IMC Proceedings 1988, 137 (1988)) . In contrast, silicon dioxide is inert toward aluminum nitride.
To increase the bonding strength between metallizing layer and the ceramic component, use has been made in the prior art of substances such as TiN, Ti, CaO, A1.sub.2 O.sub.3, Si, SiO.sub.2, Al, Ta, Cr, which form crystalline phases on the surface of the aluminum nitride and thus adhere well (EP 0 433 992 A1, EP 0 305 295 B1, EP 0 276 788 B1, U.S. Pat. No. 5,063,121 and U.S. Pat. No. 4,840,853). Such pulverulent additives are, however, very expensive, for example tantalum, titanium, titanium nitride, or the particle size of the powder can be set only with great difficulty, for example in the case of aluminum. The firing temperatures in the case of such additions are usually 1500.degree.-1700.degree. C. and are thus extraordinarily high.